Method and apparatus for merging EDA coverage logs of coverage data

ABSTRACT

An electronic design automation technology merges coverage logs. The coverage logs are generated by verification of a hardware description language circuit design. The coverage logs are merged as the coverage logs are generated, without waiting for all pending coverage logs. Another electronic design automation technology also merges coverage logs. The merged coverage logs include a first coverage log of a first simulation of a hardware description language circuit design and a second coverage log of a second simulation of the hardware description language circuit design. The first simulation is based on a first hardware verification language coverage model of the hardware description language circuit design. The second simulation is based on a second hardware verification language coverage model of the hardware description language circuit design. The second hardware verification language coverage model is newer and different than the first hardware verification language coverage model.

BACKGROUND

Electronic design automation EDA is applied in the semiconductorindustry for virtually all device design projects. After an idea for theproduct is developed, EDA tools are utilized to define a specificimplementation. The implementation defined using EDA tools is used tocreate mask data used for production of masks for lithographic use inthe production of the finished chips, in a process referred to astape-out. The masks are then created and used with fabrication equipmentto manufacture integrated circuit wafers. The wafers are diced, packagedand assembled to provide integrated circuit chips for distribution.

An exemplary procedure for design using EDA tools begins with an overallsystem design using architecture defining tools that describe thefunctionality of the product to be implemented using the integratedcircuit. Next, logic design tools are applied to create a high leveldescription based on description languages such as Verilog or VHDL, andfunctional verification tools are applied in an iterative process toassure that the high-level description accomplishes the design goals.Next, synthesis and design-for-test tools are used to translate thehigh-level description to a netlist, optimize the netlist for targettechnology, and design and implement tests that permit checking of thefinished chip against the netlist.

A typical design flow might next include a design planning stage, inwhich an overall floor plan for the chip is constructed and analyzed toensure that timing parameters for the netlist can be achieved at a highlevel. Next, the netlist may be rigorously checked for compliance withtiming constraints and with the functional definitions defined at thehigh level using VHDL or Verilog. After an iterative process to settleon a netlist and map the netlist to a cell library for the final design,a physical implementation tool is used for placement and routing. A toolperforming placement positions circuit elements on the layout, and atool performing routing defines interconnects for the circuit elements.

The components defined after placement and routing are usually thenanalyzed at the transistor level using an extraction tool, and verifiedto ensure that the circuit function is achieved and timing constraintsare met. The placement and routing process can be revisited as needed inan iterative fashion. Next, the design is subjected to physicalverification procedures, such as design rule checking DRC, layout rulechecking LRC and layout versus schematic LVS checking, that analyzemanufacturability, electrical performance, lithographic parameters andcircuit correctness.

After closure on an acceptable design by iteration through design andverify procedures, like those described above, the resulting design canbe subjected to resolution enhancement techniques that provide geometricmanipulations of the layout to improve manufacturability. Finally, themask data is prepared and taped out for use in producing finishedproducts.

SUMMARY

One aspect of the technology is an electronic design automation methodthat merges coverage logs. The coverage logs are generated byverification of a hardware description language circuit design. Thecoverage logs are merged as the coverage logs are generated, withoutwaiting for all coverage logs of pending verification. Examples ofpending verification are pending dynamic simulation (e.g., pure randomsimulation, directed random simulation, and pure directed simulation)and pending formal verification.

In some embodiments, at least one of the coverage logs is generated bysimulation of the hardware description language circuit design. In someembodiments, merging coverage logs results in a merged coverage logincluding formal verification coverage data.

Various embodiments have results that are responsive to the merging ofcoverage logs. One such result is altering a condition of a pendingsimulation of the hardware description language circuit design. Examplesf changing a condition include altering an input parameter, such asaltering an input condition (e.g., changing the input configuration fileand/or parameters to simulate the chip in a differentmode/configuration, changing the control parameters of the testbenchthat would in turn add more constraints and/or relax existingconstraints thereby causing different input stimulus to be generated) orchanging a random seed condition.

Another such result is freeing at least part of mass storage.

Another such result—responsive to not just the merging of coverage logsbut also to a predetermined condition being met—is generating a coveragereport.

Another such result is determining that, completing a pendingverification is expected to insufficiently improve verification coverageof the hardware description language circuit design. One way todetermine that the pending verification is expected to insufficientlyimprove verification coverage, is determining that an attribute of thehardware description language circuit design to be simulated by apending simulation has been simulated already. In one embodiment, whenit is determined that the pending verification is expected toinsufficiently improve verification coverage, then the pendingverification is stopped.

Another such result updating a coverage metric of verification coverageof the hardware description language circuit design. The coverage metricis inclusive of coverage data from merging the coverage logs. Examplecoverage metrics consider assertion coverage, functional coverage, andcode coverage (e.g., line coverage, condition coverage, branch coverage,path coverage, toggle coverage, assign toggle coverage).

In some embodiments, the technology changes from a status ofunreadiness, to readiness to merge coverage logs. In response to thisstatus change, coverage log data are requested that was generated priorto the status change.

Some embodiments create multiple running instances. These multiplerunning instances perform merging of coverage logs of simulations of thehardware description language circuit design, or verification generally.

Another aspect of the technology is an electronic design automationmethod that merges coverage logs. The merged coverage logs include afirst coverage log of a first simulation of a hardware descriptionlanguage circuit design and a second coverage log of a second simulationof the hardware description language circuit design. The firstsimulation is based on a first hardware verification language coveragemodel of the hardware description language circuit design. The secondsimulation is based on a second hardware verification language coveragemodel of the hardware description language circuit design. The secondhardware verification language coverage model is newer and differentthan the first hardware verification language coverage model.

The hardware description language of the hardware description languagecircuit design includes any of Verilog, SystemVerilog, and VHDL.

The hardware verification language of the first and second hardwareverification language models includes any of SystemVerilog, NativeTestbench, E, and Vera.

Various embodiments have various conditions that control whethercoverage data is deleted and/or retained, responsive to merging thecoverage logs.

One such condition includes, responsive to the first coverage log andthe second coverage log including different maximum numbers ofautomatically created bins, retaining coverage data of the cover pointof the second coverage log and deleting coverage data of the cover pointfrom the first coverage log.

Another such condition includes, responsive to the first coverage logincluding a bin having a bin name absent from the second coverage log,deleting coverage data of the bin after said merging.

Another such condition includes, responsive to the first coverage logincluding a bin having a bin name present in the second coverage log,retaining coverage data of the bin after said merging.

Another such condition includes, responsive to the first coverage loghaving a cover point with a first expression width and the secondcoverage log having the cover point with a second expression widthdifferent from the first expression width, retaining coverage data ofthe cover point with the second expression width and deleting coveragedata of the cover point with first expression width.

Another such condition includes, responsive to the first coverage loghaving a bin with a first bin definition and the second coverage log hasthe bin with a second bin definition different from the first bindefinition, then after said merging, retaining coverage data of the binwith the second bin definition and deleting coverage data of the firstbin definition.

Another such condition includes, responsive to the first coverage logincluding a cross coverage point name of a cross coverage point namingat least two coverage point identifiers, and the second coverage logincludes the cross coverage point name of the cross coverage pointnaming said at least two coverage point identifiers, then deletingcoverage data of the cross coverage point of the first coverage logresponsive to deletion of coverage data of at least one of theidentified coverage points from the first coverage log.

Another such condition includes, responsive to the first coverage logincluding a first user-defined cross bin of a first cross coveragepoint, and the second coverage log including a second user-defined crossbin of a second cross coverage point, the first cross coverage point andthe second cross coverage point having a same name, then despite thefirst user-defined cross bin and the second user-defined cross binhaving different definitions, retaining coverage data of the firstuser-defined cross bin and coverage data of the second user-definedcross bin.

Another such condition includes, responsive to the first coverage logincluding first autocross bins of a first cross coverage point, and thesecond coverage log including a second user-defined cross bin subsumingthe autocross bins of the first cross coverage point, retaining coveragedata of the first autocross bins in the second user-defined cross bin ofthe second coverage log.

Another such condition includes, responsive to the first coverage logincluding a first cross coverage point, and the second coverage logincluding a second cross coverage point, then despite the first crosscoverage point and the second cross coverage point having different autocross coverage space, retaining i) auto cross coverage data of the firstcross coverage point that is subsumed by auto cross coverage space ofthe second cross coverage point, and ii) coverage data of the secondcross coverage point.

Another such condition includes, responsive to the second coverage logincluding a cross coverage point absent from the first coverage log,retaining the cross coverage point.

Another such condition includes, responsive to the first coverage logincluding a cross coverage point absent from the second coverage log,deleting the cross coverage point.

Another such condition includes, responsive to the first coverage logincluding property coverage absent from the second coverage log,retaining the property coverage.

Another such condition includes, responsive to the second coverage logincluding property coverage absent from the first coverage log,retaining the property coverage.

Other aspects of the technology are directed to a computer readablemedium with computer readable instructions for circuit design, includingcomputer instructions performing the technology described herein. Otheraspects of the technology are directed to a computer with a memorystoring computer instructions performing the technology describedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified process flow of making integrated circuits, fromthe idea stage, through electronic design automation (EDA) stages, andending with the integrated circuits.

FIG. 2 is a simplified block diagram of a system performing simulationof a hardware verification language circuit design.

FIG. 3 is a simplified state diagram showing the operation of thesimulation system of FIG. 2.

FIG. 4 is a simplified directory structure of the simulation system ofFIGS. 2 and 3.

FIGS. 5A and 5B are simplified flow charts contrasting an improvedsimulation flow that merges, rather than discards, prior coverage logs.

FIG. 6 is a simplified block diagram of a computer system of variousembodiments.

DETAILED DESCRIPTION

FIG. 1 is a simplified process flow of making integrated circuits, fromthe idea stage, through electronic design automation (EDA) stages, andending with the integrated circuits.

FIG. 1 shows a simplified representation of an illustrative digitalintegrated circuit design and test flow. As with all flowcharts herein,it will be appreciated that many of the steps in FIG. 1 can be combined,performed in parallel or performed in a different sequence withoutaffecting the functions achieved. In some cases a re-arrangement ofsteps will achieve the same results only if certain other changes aremade as well, and in other cases a re-arrangement of steps will achievethe same results only if certain conditions are satisfied. Suchre-arrangement possibilities will be apparent to the reader.

At a high level, the process of FIG. 1 starts with the product idea(step 100) and is realized in an EDA (Electronic Design Automation)software design process (step 110). When the design is finalized, thefabrication process (step 150) and packaging and assembly processes(step 160) occur resulting, ultimately, in finished integrated circuitchips (result 170). Some or all of the finished chips are tested in step180 on a tester machine using predefined test vectors and expectedresponses.

The EDA software design process (step 110) is actually composed of anumber of steps 112-130, shown in linear fashion for simplicity. In anactual integrated circuit design process, the particular design mighthave to go back through steps until certain tests are passed. Similarly,in any actual design process, these steps may occur in different ordersand combinations. This description is therefore provided by way ofcontext and general explanation rather than as a specific, orrecommended, design flow for a particular integrated circuit.

A brief description of the components steps of the EDA software designprocess (step 110) will now be provided.

System design (step 112): The designers describe the functionality thatthey want to implement, they can perform what-if planning to refinefunctionality, check costs, etc. Hardware-software architecturepartitioning can occur at this stage. Example EDA software products fromSynopsys, Inc. that can be used at this step include Model Architect,Saber, System Studio, and DesignWare® products.

Logic design and functional verification (step 114): At this stage, theVHDL or Verilog code for modules in the system is written and the designis checked for functional accuracy. More specifically, the design ischecked to ensure that it produces the correct outputs in response toparticular input stimuli. Example EDA software products from Synopsys,Inc. that can be used at this step include VCS, VERA, DesignWare®,Magellan, Formality, ESP and LEDA products. While some designs might atthis stage already include certain design-for-test features such as scanchains and associated scan compression or decompression circuitry, theseare not included in the terms “logic design” and “circuit design” asthey are used herein.

Synthesis and design for test (DFT) (step 116): Here, the VHDL/Verilogis translated to a netlist. The netlist can be optimized for the targettechnology. Additionally, the implementation of a test architectureoccurs in this step, to permit checking of the finished chips. ExampleEDA software products from Synopsys, Inc. that can be used at this stepinclude Design Compiler®, Physical Compiler, Test Compiler, PowerCompiler, FPGA Compiler, TetraMAX, and DesignWare® products. A currentproduct for implementing a test architecture, with a few user-specifiedconfiguration settings as described above, is DFT MAX. DFT MAX isdescribed in Synopsys, DFT MAX Adaptive Scan Compression Synthesis,Datasheet (2007), incorporated herein by reference.

Netlist verification (step 118): At this step, the netlist is checkedfor compliance with timing constraints and for correspondence with theVHDL/Verilog source code. Example EDA software products from Synopsys,Inc. that can be used at this step include Formality, PrimeTime, and VCSproducts.

Design planning (step 120): Here, an overall floor plan for the chip isconstructed and analyzed for timing and top-level routing. Example EDAsoftware products from Synopsys, Inc. that can be used at this stepinclude Astro and IC Compiler products.

Physical implementation (step 122): The placement (positioning ofcircuit elements) and routing (connection of the same) occurs at thisstep. Example EDA software products from Synopsys, Inc. that can be usedat this step include the Astro and IC Compiler products.

Analysis and extraction (step 124): At this step, the circuit functionis verified at a transistor level, this in turn permits what-ifrefinement. Example EDA software products from Synopsys, Inc. that canbe used at this step include AstroRail, PrimeRail, Primetime, and StarRC/XT products.

Physical verification (step 126): At this step various checkingfunctions are performed to ensure correctness for: manufacturing,electrical issues, lithographic issues, and circuitry. Example EDAsoftware products from Synopsys, Inc. that can be used at this stepinclude the Hercules product.

Tape-out (step 127): This step provides the “tape-out” data forproduction of masks for lithographic use to produce finished chips.Example EDA software products from Synopsys, Inc. that can be used atthis step include the CATS(R) family of products.

Resolution enhancement (step 128): This step involves geometricmanipulations of the layout to improve manufacturability of the design.Example EDA software products from Synopsys, Inc. that can be used atthis step include Proteus, ProteusAF, and PSMGen products.

Mask preparation (step 130): This step includes both mask datapreparation and the writing of the masks themselves.

Aspects of the present technology relate particularly to “logic designand functional verification” described above.

Design and functional verification is the process of proving that thehardware description language circuit design satisfies the designspecification of the circuit. Although both static and dynamic methodsof verification are available, static methods tend to be limited torelatively small circuit blocks due to the limitations of performingformal proofs of the hardware description language circuit design.Dynamic methods rely on simulation of the hardware description languagecircuit design, and cannot achieve complete verification. To accomplisha reasonably thorough verification of the hardware description languagecircuit design with a modern complexity of millions of gates, simulationfarms run numerous simulations with different conditions, such asdifferent input conditions or different random seeds. Such simulationcontinues until the aggregate coverage metric(s) is/are satisfactory.

Due to the trend of decreasing cost for increased computation resources,each time the simulation model is adjusted, one of ordinary skill in theart would tend to re-run simulations to reach a satisfactory coveragemetric(s) result.

The resulting coverage data from completed simulations of the hardwaredescription language circuit design are stored in mass storage ascoverage logs. The various Electronic Design Automation vendorsgenerally have their own proprietary coverage logs that store onlyElectronic Design Automation coverage data and do not store generalpurpose data as with databases by Oracle, IBM, MySQL, etc. Coverage datais a set of interesting conditions, or attributes, that are expected tohappen during simulation.

In some embodiments, the coverage logs are merged. The coverage model isa set of interesting conditions that we want to observe duringsimulation. Each simulation creates a coverage log that represents whichportion of the coverage model was covered by that simulation. Themerging operation aggregates the coverage logs to compute which parts ofthe coverage model are collectively covered by all the simulations.

The simulations are performed according to the hardware verificationlanguage coverage model, which includes functional coverage andassertion coverage. Other embodiments include various combinations ofone or more of functional coverage, assertion coverage, and codecoverage.

Coverage data are discussed in “IEEE Standard for SystemVerilog—UnifiedHardware Design, Specification, and Verification Language”, IEEE Std1800, hereby incorporated by reference. The following discussion ofcoverage data is largely taken from this IEEE standard.

A coverage group, or covergroup, can contain one or more coveragepoints, or coverpoints. A coverage point can be an integral variable oran integral expression. Each coverage point includes a set of binsassociated with its sampled values or its value transitions. The binscan be explicitly defined by the user or automatically created by thehardware verification language environment. Bins for a coverage pointcan be automatically created by the hardware verification languageenvironment or explicitly defined.

A coverage point bin, or bin, associates a name and a count with a setof values or a sequence of value transitions. If the bin designates aset of values, the count is incremented every time the coverage pointmatches one of the values in the set. If the bin designates a sequenceof value transitions, the count is incremented every time the coveragepoint matches the entire sequence of value transitions.

Thus, a bin is a unit identifying an attribute of the hardwaredescription language circuit undergoing simulation, such that aftersimulation the coverage metric of the model is calculated based on thebins. The coverage metric is an assessment of progress of verifying ahardware description language circuit design. The coverage model is theaggregate of all bins, including cross points of bins and cover pointsof bins. In some embodiments, the same model is used for allsimulations, and the resulting coverage data are merged. In otherembodiments, different models are used for at least some simulations,and the resulting coverage data are selectively merged.

A cover group can specify cross coverage between two or more coveragepoints or variables. Cross coverage of a set of N coverage points isdefined as the coverage of all combinations of all bins associated withthe N coverage points, that is, the Cartesian product of the N sets ofcoverage point bins. An autocross coverage is an automatically generatedcross coverage of a coverage point with another coverage point.

Coverage of a coverage point item can be computed differently dependingon whether the bins of the coverage point are explicitly defined by theuser or automatically created by the tool.

For user-defined bins, the coverage of a coverage point is computed asthe ratio of: i) the cardinality of the covered bins, i.e. the subset ofall (defined) bins that are covered, and ii) the cardinality of the setof bins defined.

For automatically generated bins, the coverage of a coverage point iscomputed as the ratio of: i) the cardinality of the covered bin, i.e.the subset of all (auto-defined) bins that are covered, and ii) thelesser of a) the minimum number of bits needed to represent the coveragepoint and b) auto_bin_max.

Cumulative coverage considers the union of all significant bins; thus,it includes the contribution of all bins (including overlapping bins) ofall instances.

To determine whether a particular bin of a coverage group is covered,the cumulative coverage computation considers the value of the at leastoption of all instances being accumulated. Consequently, a bin is notconsidered covered unless its hit count equals or exceeds the maximum ofall the at_least values of all instances. Use of the maximum representsthe more conservative choice.

FIGS. 2-4 address an embodiment of EDA verification, in particularcoverage driven verification.

This discussed embodiment does not require that all coverage logs becreated before they can be merged. This approach scales particularlywell in a random regression test environment where random tests are run24 hours a day, 7 days a week, with numerous input conditions or randomseeds. Because coverage logs are merged prior to the creation of thelast coverage log, in this approach the amount of disk space required isno longer directly proportional to the number of tests. Similarly, onedoes not need to wait until the last simulation has finished running, tosample a merged coverage log. A pipeline is maintained of theunprocessed (unmerged) coverage logs. Early aggregation of coverage logsalso allows assists the identification and analysis of coverage holeswell before all the tests have finished running. “Bad” stimulus isuncovered sooner rather than later.

FIGS. 2-4 are discussed below.

FIG. 2 is a simplified block diagram of a system performing simulationof a hardware verification language circuit design. The system includesindividual simulation clients 210, a file system 220 to store coveragelogs, a location repository server 230, and merge daemons 240. Thesimulation clients acting as multiple producers, and “urg” merge daemonsact as multiple consumers and provide the synchronization primitives tomaintain consistency of the queue.

Individual simulation clients 210 are the individual random simulationsthat generate functional coverage logs. Each test client dumps out acoverage log representing parts of the coverage model it targeted. Eachclient ensures the following:

1) It does not overstep on a different simulation client's databasedirectory.

2) It saves the log on a file system accessible from the machine wherethe “Urg merge daemons” are running for unified report generators.

3) These clients talk to the location repository server using aninterface, for example in Perl.

The clients inform the location repository server 230 of the directorywhere the coverage logs are located on the file system 220. An exampleimplementation is an object oriented perl interface e.g.VtgSimClientIntf, with the following examplary methods:

1) new: Constructor to create the object. Takes two mandatory argumentsand one optional argument.

i) server: name of the machine where the location repository server isrunning

ii) port: port number on which the location repository server is running

iii) newmodel: This argument is optional. The default value is 0. Whenset to 1, it switches the master model used for flexible merging at themerge server.

2) dispatchLocation: Method to inform the location repository serverregarding the location of the coverage log. Returns a small xml stringon success and undef on failure.

Example 1:

use VtgSimClientIntf; my $simcli =VtgSimClientIntf->new(server=>‘vgamd84’, port=>7070);  my $retVal =$simcli ->dispatchDbLocation(“/u/foo/bar.vdb”); If successful the valueof $retVal would be: <dbsummary currdb=“/u/foo/bar.vdb” />

Example 2:

  use VtgSimClientIntf;   my $simcli =VtgSimClientIntf->new(server=>‘vgamd84’, port=>7070, newmodel=>1);    my$retVal = $simcli ->dispatchDbLocation(“/u/foo/bar.vdb”);

This example switches the master coverage model used by mergeserver tothe one stored under/u/foo/bar.vdb. If successful the value of $retValwould be:

<dbsummary currdb=“/u/foo/bar.vdb”/>

The file system 220 to hold coverage logs collectively representsenormous amounts of mass storage, due to the demands of storing thecoverage logs from the large number of individual simulation clients210.

Location repository server 230 is a lightweight server which keeps trackof the location of individual coverage log directories generated byindividual simulations. It keeps track of the top directory location.Some embodiments do not do data movement.

Merge daemons 240 are computation intensive processes that query thelocation repository server 230 to fetch the list of coverage logs toprocess. The actual coverage logs are then accessed from the file system220 and processed. An aggregation tool such as urg (unified reportgenerator) merges coverage logs generated by individual simulations tocompute which parts of the coverage model are collectively covered byall tests in the verification suite. The merge daemons provide hooks tosample the accumulated coverage logs before completion of all coveragelogs.

FIG. 3 is a simplified state diagram showing the operation of thesimulation system of FIG. 2.

The early state is start merge server 310. The merge server starts andis capable of receiving coverage logs, and then fires the‘serv_post_start_call_back’. This callback is used for submitting anycoverage log that has been generated while the merge server was offline.If multiple merge servers run on the same machine, option“daemon_port_start” specifies the port of the merge server behind whichthe merge daemons run.

The intermediate state is active state 310. As simulations run, when aparticular simulation job finishes, it uses the simulation clientinterface to submit the coverage log location to the merge server. Atthis state, checkpoint callbacks occur. After merging a predeterminednumber of coverage logs (configurable by checkpoint_granularity in theconfig file) or at regular intervals (configurable by time_granularity),it fires the ‘checkpoint callback’ for each merge daemon. This callbackdeletes coverage logs which do not add any value or keep track of howthe coverage metrics are changing.

The later state is stop the merge server 330. At this point it firesdaemon_post_call_back for each merge daemon. This performs clean upactivities, sends email etc.

Details regarding merge server hooks/callbacks follow below, listed fromA)-E):

A) Coverage log submission hook: This hook allows coverage logs tosubmitted to the merge server for aggregation.

B) Location server post start callback: (‘serv_post_start_call_back’ inthe diagram) This callback gets fired once the merge server starts andis capable of receiving coverage logs. This callback should be used forsubmitting any coverage logs (using the coverage log submission hook)that have been generated while the merge server was offline.

C) Checkpoint callback: (checkpoint_callback in the diagram) Thiscallback gets fired at regular intervals, configured via the followingparameters:

1) time_granularity: The user can set this parameter to a value (inseconds) to indicate when this callback should be fired. E.g if thevalue is 1800 sec, the callback will be fired after every 1800 sec.

2) checkpoint_granularity: The user can set this parameter to indicatehow often this callback should be fired. For e.g. if the value is set to900, then this callback will be fired after merging every 900 coveragelogs.

In addition to the predetermined conditions above, other predeterminedconditions that fire this callback and generate a coverage report are acoverage percentage or other coverage metric amount reached by themerged coverage log.

If both the parameters are set, then the callback will be firedwhichever event happens first. The checkpoint_callback has the followinginfo:

1) Base score for each metric in the last callback

2) For each coverage log merged, the checkpoint callback has thefollowing info: 2a) name of the coverage log 2b) aggregated coveragescore for each metric

This callback can be used to implement custom actions. Some of them arementioned below:

1) Incremental grading: If the incremental coverage score for a coveragelog is zero, then it can be deleted. Incremental coverage added by atest is built into the hooks exposed by the merge server. So, if acoverage log does not add any value it can be deleted promptly, therebybounding the required amount of mass storage by about the total numberof coverable objects being targeted.

2) Monitor how coverage score is changing with time: This callback canbe used to plot how the coverage score is changing with time. If thecoverage score does not change or changes very little, the client canattempt to change the stimulus or indicate to a real person (viaemail/page) that an action needs to be taken.

3) Backing up of good tests: The client can determine if a specificcoverage log indicates a change in coverage score or covers a difficultto hit/cover coverable object and back up that test.

Generally, checkpoint callback command is capable of runningasynchronously with the merge daemon. The merge daemon will not wait forthe script to finish. The checkpoint callback interface is a commandcapable of processing an xml file. If the command is ‘cpcb’, it would beinvoked as ‘cpcb path-to-a-valid-xml-file’ from the merge daemon afterit creates a checkpoint.

An example schema of the checkpoint callback interface file follows:

<!ELEMENT checkpointed_databases (basescore) (dir*)> <!ELEMENT basescore(score*) > <!ATTLIST basescore EMPTY> <!ELEMENT dir (test*) ><!ATTLIST dir path CDATA #REQUIRED> <!ATTLIST dir error (yes|no)#REQUIRED> <!ATTLIST dir newmodel (yes|no) #OPTIONAL> <!ELEMENT test(score*) > <!ATTLIST test name CDATA #REQUIRED> <!ELEMENT score ><!ATTLIST score metric (assert|testbench|line|cond|fsm|branch) #REQUIRED > <!ATTLIST score numerator CDATA #REQUIRED> <!ATTLISTscore denominator CDATA #REQUIRED>

The “basescore” element contains the coverage data score for the testonto which all the coverage log directories were incrementally merged.

The “dir” element represents the top level coverage log directory thatwas processed. The attribute “path” indicates the location of thedirectory. The attribute “error” indicates if an error occurred whileprocessing that directory. The error details are redirected to file“err.log”.

“score” represents the coverage score for each test on a metric bymetric basis.

An example checkpoint callback file follows:

<checkpointed_databases>  <basescore >   <score metric=“assert”numerator=“0” denominator=“0” />   <score metric=“testbench”numerator=“0” denominator=“0” />  </basescore>  <dirpath=“/remote/vtghome6/manojb/demo/pr/junk/db_0” , error=“no”>   <testname=“/remote/vtghome6/manojb/demo/pr/junk/db_0/results” >   <scoremetric=“assert” numerator=“1” denominator=“1” />   <scoremetric=“testbench” numerator=“0” denominator=“0” />   </test>   <testname=“/remote/vtghome6/manojb/demo/pr/junk/db_0/t3” >   <scoremetric=“assert” numerator=“1” denominator=“1” />   <scoremetric=“testbench” numerator=“0” denominator=“0” />   </test>   <testname=“/remote/vtghome6/manojb/demo/pr/junk/db_0/test” >   <scoremetric=“assert” numerator=“1” denominator=“1” />   <scoremetric=“testbench” numerator=“47” denominator=“50997” />   </test> </dir>  <dir path=“/remote/vtghome6/manojb/demo/pr/junk/db_1” ,error=“no” newmodel=“yes”>   <testname=“/remote/vtghome6/manojb/demo/pr/junk/db_1/results” >   <scoremetric=“assert” numerator=“1” denominator=“1” />   <scoremetric=“testbench” numerator=“47” denominator=“50997” />   </test>  <test name=“/remote/vtghome6/manojb/demo/pr/junk/db_1/t3” >   <scoremetric=“assert” numerator=“1” denominator=“1” />   <scoremetric=“testbench” numerator=“47” denominator=“50997” />   </test>  <test name=“/remote/vtghome6/manojb/demo/pr/junk/db_1/test” >   <scoremetric=“assert” numerator=“1” denominator=“1” />   <scoremetric=“testbench” numerator=“47” denominator=“50997”/>   </test> </dir> </ checkpointed_databases>

D) Daemon pre start call back: (daemon_pre_call_back in the diagram).This callback is fired once before the merge daemon start processingcoverage logs. This callback can be used to collect statistics for eachmerge daemon.

E) Daemon post start call back: (daemon_post_call_back in the diagram)This callback gets fired when the merge server is stopped. One suchcallback is fired for each merge daemon. This can be used to takeactions like the following:

1. Generate custom reports

2. Send emails etc.

FIG. 4 is a simplified directory structure of the simulation system ofFIGS. 2 and 3.

By default each merge daemon creates a sub directory inside thedirectory specified by directory_prefix. For example, for the defaultconfiguration file specified by “./aggregated_data”, the directorystructure of FIG. 4 is created. “UrgDaemon_(—)1/UserDir/MergeDb” is theaggregated coverage log created by the first merge daemon.“UrgDaemon_(—)1/UserDir/err.log” is the error log file for the coveragelog aggregation done by the first daemon. Errors (corrupt coverage logs,bugs/crashes in the tool) are redirected to this file.

The following is an example syntax of launching a merge server onlocation repository server 230.

mergeserv [--config=<config_file>]--start|--stop

config_file is the [optional] configuration file read by the mergeserver. An example format of this configuration file is provided below.

--start: Starts the merge server. If the merge server is alreadyrunning, this option does not do anything.

--stop: Stops the merge server. This initiates the stop sequence. Themerge server exits after all the submitted databases have been merged.

An example merge server configuration file follows, in xml format.

 <mserv_config>   <location_serv port=″7070″datadir=”/u/regress_usr/locationdb” />   <serv_post_start_call_backcommand=”submit_again” />   <merge_daemon num=″2″directory_prefix=”./aggregated_data”>    <daemon_pre_call_backcommand=”pre.pl”/>     <merge_daemon_optionscheckpoint_granularity=”300” time_granularity=“900” />    <check_point_call_back command=”foo.pl” />    <daemon_post_call_back command=”post.pl”/>   </merge_daemon> </mserv_config>

In this example default configuration:

1) Location repository server runs on port 7070.

2) A single instance of merge daemon is fired.

3) All merge daemon clients create the aggregated data under directory“aggregated_data”.

4) Merge server maintains the list of unprocessed coverage logs in a logthat is stored under the directory specified by the attribute datadir.In many embodiments, only the user running the merge server has writepermissions to this directory. The default value of datadir is “.”.

Running multiple instances of the merge daemon is a performanceoptimization to handle the case in which coverage logs are generatedquickly. To run multiple merge servers on the same machine, the configfile used for firing the individual merge servers should not conflictwith each other. Specifically the following entries in the config fileshould not conflict.

i) location serv port

ii) datadir

iii) directory_prefix

iv) daemon_port_start

Also, any callback script used with the merge servers should not modifythe same system resources. For example, if the checkpoint callback savesuseful test information in some file/directory, then the file/directoryused by the individual callback scripts should be different for eachmerge server.

Here is an example of sample config files for running two merge serverson the same machine.

<mserv_config>   <location_serv port=″7070″datadir=”/u/regress_usr/locationdb” />   <serv_post_start_call_backcommand=”submit_again” />   <merge_daemon num=″2″directory_prefix=”./aggregated_data”>    <daemon_pre_call_backcommand=”pre.pl”/>     <merge_daemon_optionscheckpoint_granularity=”300” time_granularity=″900” />    <check_point_call_back command=”foo.pl” />    <daemon_post_call_back command=”post.pl”/>   </merge_daemon> </mserv_config>  <mserv_config>   <location_serv port=″7071″datadir=”/u/regress_usr/locationdb.second”/>  <serv_post_start_call_back command=”submit_again” />   <merge_daemonnum=″2″ directory_prefix=”./aggregated_data.second”>   <daemon_pre_call_back command=”pre.pl”/>     <merge_daemon_optionscheckpoint_granularity=”300” time_granularity=“900”daemon_port_start=”14449” />     <check_point_call_back command=”foo.pl”/>     <daemon_post_call_back command=”post.pl”/>   </merge_daemon></mserv_config>

Whereas the preceding text is generally directed to merging coveragelogs, the following text is generally directed to techniques of mergingparticular coverage logs.

FIGS. 5A and 5B are simplified flow charts contrasting an improvedsimulation flow that merges, rather than discards, prior coverage logs.Due to the trend of decreasing cost for increased computation resources,each time the simulation model is adjusted, one of ordinary skill in theart would tend to re-run simulations to reach a satisfactory coveragemetric(s) result, as shown in FIG. 5A, rather than preserving anycoverage data from coverage logs of old simulation models.

The process flow of FIG. 5A is as follows. In 501, a functional coveragemodel is created. In 503, random simulations are run, starting from someinput condition or some first random seed. In 505, coverage logsgenerated by simulations are merged. In 507, the merged coverage log isanalyzed. In 509, the functional coverage model is refined. The processflow repeats, after 611, in which the previous coverage logs aredeleted.

The process flow of FIG. 5B modifies the process flow of FIG. 5A. In504, rather than starting the random simulations from some inputcondition or some first random seed as in 503, the random simulationsare run with new input conditions or new random seeds. In 512, ratherthan deleting the coverage logs as in 511, the previous coverage logsare imported.

The technology of FIG. 5B can be applied to either the case wherecoverage logs are merged when all simulations are completed, or prior tocompletion of the last simulation as discussed above in connection withFIGS. 2-4.

The following examples use SystemVerilog style syntax to representcoverage logs, but are applicable to any hardware verification languagewith comparable syntax. Log T1 represents an older coverage log. Log T2represents a newer coverage log. The Merged Log represents the result ofmerging the respective coverage logs. Each example is followed byexplanation.

Example 1:

Log T1 Log T2 Merged Log cp1: coverpoint firstsig; cp1: coverpointfirstsig; cp1: coverpoint firstsig; option.auto_bin_max = 64;option.auto_bin_max = 32; option.auto_bin_max = 32;

According to Example 1, differing auto_bin_max values are not mergeequivalent.

Example 2:

Log T1 Log T2 Merged Log cp2: coverpoint cp2: coverpoint cp2: coverpointsecondsig { secondsig { secondsig { bin first = [0:63]; bin first =[0:63]; bin first = [0:63]; bin mid = [71:82]; bin second = [65:128];bin second = [65:128]; } } }

According to Example 2, “bin mid=[71:82]” of “coverpoint secondsig” ofLog T1 is deleted because it is missing in the definition of “coverpointsecondsig” in Log T2.

Example 3:

Log T1 Log T2 Merged Log cp3: coverpoint thirdsig; cp3: coverpointthirdsig; cp3: coverpoint thirdsig;

According to Example 3, cp3 of log T1 is merge equivalent to cp3 of logT2 and they are merged.

Example 4:

Log T1 Log T2 Merged Log Bit[7:0]signal; Bit[15:0]signal; cp4:coverpoint signal; cp4: coverpoint signal; cp4: coverpoint signal;

According to Example 4, “cp4: coverpoint signal” of Log T1 is deletedbecause the width of the coverpoint expression (‘signal’) has changedfrom 8 bits to 16 bits.

Example 5:

Log T1 Log T2 Merged Log cc1: cross cp1, cp2; cc1: cross cp1, cp2; cc1:cross cp1, cp2;

According to Example 5, “cc1: cross cp1, cp2;” of Log T1 is deleted dueto coverpoint “cp1” of Log T1 being deleted (refer to example 1 for cp1)

Example 6:

Log T1 Log T2 Merged Log cc2: cross cp2, cp3 { cc2: cross cp2, cp3 {cc2: cross cp2, cp3 { bins mybin = bins mybin = bins mybin = binsof(cp2)intersect binsof(cp2) intersect binsof(cp2) intersect [0:255]; [0:255];[0:255]; } bins yourbin = bins yourbin = binsof(cp2) intersectbinsof(cp2) intersect [256:511]; [256:511]; } }

According to Example 6, the coverage data for user bins mybin is mergedfor T1 and T2. The auto crosses of cc2 in T1 which are also in the autocross coverage space of cc2 in T2 are merged.

Example 7:

Log T1 Log T2 Merged Log cc3: cross cp2, cp3 { cc3: cross cp2, cp3 {cc3: cross cp2, cp3 { bins mybin = bins mybin = bins mybin = binsof(cp2)intersect binsof(cp2) intersect binsof(cp2) intersect [0:255]; [0:8191];[0:255]; } } bins mybin = binsof(cp2) intersect [0:8191]; }

According to Example 7, autocrosses seen in log T1 will still beimported and merged with the ones seen in log T2, despite the fact thatautocross coverage space of cc3 is different in T1 and T2 due todiffering definitions of the user bin ‘mybin’. Some of the autocrossbins of cc3 in T1 are subsumed in the definition of user bin mybin inlog T2. These autocross bins from log T1 are merged into user bin mybinof log T2. Remaining autocross bins in log T1 will be merged withcorresponding autocross bins in log T2.

Example 8:

Log T1 Log T2 Merged Log cc4: cross cp1, cp2, cc4: cross cp1, cp2, cp3cp3

According to Example 8, cross point cc4 absent from log T1 but presentin T2 is retained after merging.

Example 9:

Log T1 Log T2 Merged Log first_prop:cover first_prop:coverfirst_prop:cover property(.......); property(.......);property(.......); second_prop:cover third_prop:cover second_prop:coverproperty(.......); property(.......); property(.......);third_prop:cover property(.......);

According to Example 7, cover property ‘third_prop’ absent from log T1but present in T2 is retained after merging; and cover property‘second_prop’ absent from log T2 but present in T1 is retained aftermerging.

FIG. 6 is a simplified block diagram of a computer system of variousembodiments.

FIG. 6 is a simplified block diagram of a computer system 610 that canbe used to implement software incorporating aspects of the presentinvention. While the flow charts and other algorithms set forth hereindescribe series of steps, it will be appreciated that each step of theflow chart or algorithm can be implemented by causing a computer systemsuch as 610 to operate in the specified manner.

Computer system 610 typically includes a processor subsystem 614 whichcommunicates with a number of peripheral devices via bus subsystem 612.Processor subsystem 614 may contain one or a number of processors. Theprocessor subsystem 614 provides a path for the computer system 610 toreceive and send information described herein, including within theprocessor subsystem 614, such as with a multi-core, multiprocessor,and/or virtual machine implementation. The peripheral devices mayinclude a storage subsystem 624, comprising a memory subsystem 626 and afile storage subsystem 628, user interface input devices 622, userinterface output devices 620, and a network interface subsystem 616. Theinput and output devices allow user interaction with computer system610. Network interface subsystem 616 provides an interface to outsidenetworks, including an interface to communication network 618, and iscoupled via communication network 618 to corresponding interface devicesin other computer systems. Communication network 618 may comprise manyinterconnected computer systems and communication links. Thesecommunication links may be wireline links, optical links, wirelesslinks, or any other mechanisms for communication of information. Whilein one embodiment, communication network 618 is the Internet, in otherembodiments, communication network 618 may be any suitable computernetwork. The communication network 618 provides a path for the computersystem 610 to receive and send information described herein.

The physical hardware component of network interfaces are sometimesreferred to as network interface cards (NICs), although they need not bein the form of cards: for instance they could be in the form ofintegrated circuits (ICs) and connectors fitted directly onto amotherboard, or in the form of macrocells fabricated on a singleintegrated circuit chip with other components of the computer system.

User interface input devices 622 may include a keyboard, pointingdevices such as a mouse, trackball, touchpad, or graphics tablet, ascanner, a touch screen incorporated into the display, audio inputdevices such as voice recognition systems, microphones, and other typesof input devices. In general, use of the term “input device” is intendedto include all possible types of devices and ways to input informationinto computer system 610 or onto computer network 618.

User interface output devices 620 may include a display subsystem, aprinter, a fax machine, or non visual displays such as audio outputdevices. The display subsystem may include a cathode ray tube (CRT), aflat panel device such as a liquid crystal display (LCD), a projectiondevice, or some other mechanism for creating a visible image. Thedisplay subsystem may also provide non visual display such as via audiooutput devices. In general, use of the term “output device” is intendedto include all possible types of devices and ways to output informationfrom computer system 610 to the user or to another machine or computersystem.

Storage subsystem 624 stores the basic programming and data constructsthat provide the functionality of certain embodiments of the presentinvention. For example, the various modules implementing thefunctionality of certain embodiments of the invention may be stored instorage subsystem 624. These software modules are generally executed byprocessor subsystem 614.

Memory subsystem 626 typically includes a number of memories including amain random access memory (RAM) 630 for storage of instructions and dataduring program execution and a read only memory (ROM) 632 in which fixedinstructions are stored. File storage subsystem 628 provides persistentstorage for program and data files, and may include a hard disk drive, afloppy disk drive along with associated removable media (illustrativelyshown as computer readable medium 640 storing circuit design 680), aCD/DVD ROM drive, an optical drive, or removable media cartridges. Thedatabases and modules implementing the functionality of certainembodiments of the invention may have been provided on a computerreadable medium such as one or more CD/DVD-ROMs, and may be stored byfile storage subsystem 628. The host memory 626 contains, among otherthings, computer instructions which, when executed by the processorsubsystem 614, cause the computer system to operate or perform functionsas described herein. As used herein, processes and software that aresaid to run in or on “the host” or “the computer”, execute on theprocessor subsystem 614 in response to computer instructions and data inthe host memory subsystem 626 including any other local or remotestorage for such instructions and data.

Bus subsystem 612 provides a mechanism for letting the variouscomponents and subsystems of computer system 610 communicate with eachother as intended. Although bus subsystem 612 is shown schematically asa single bus, alternative embodiments of the bus subsystem may usemultiple busses.

Computer system 610 itself can be of varying types including a personalcomputer, a portable computer, a workstation, a computer terminal, anetwork computer, a television, a mainframe, a parallel processingsystem, a network of more than one computer, or any other dataprocessing system or user device. Due to the ever changing nature ofcomputers and networks, the description of computer system 610 depictedin FIG. 6 is intended only as a specific example for purposes ofillustrating the preferred embodiments of the present invention. Manyother configurations of computer system 610 are possible having more orless components than the computer system depicted in FIG. 6.

The following is a mathematical appendix discussing exemplary conditionsof merging coverage logs.

1. Merge Equivalence

Coverable objects across logs are merged if they are merge equivalent.The notion of merge equivalence is different for coverpoints andcrosspoints. We use the notation A1≡_(merge-eq) A2 to indicate that A1and A2 are merge equivalent.

1.1 Merge Equivalence of Coverpoints

1.1.1 Merge Equivalence of Autobinned Coverpoints

1.1.1.1. Default Merge Equivalence

P1≡_(merge-eq) P2 iff

-   -   name(P1)≡name(P2)    -   auto_bin_max(P1)=auto_bin_max(P2)    -   width(P1)=width(P2)

where P1 and P2 are auto-binned coverpoints.

1.1.1.2 Flexible Merge Equivalence

This is same as default merge equivalence for autobinned coverpoints.

1.1.2 Merge Equivalence of Userbinned Coverpoints

1.1.2.1 Default Merge Equivalence

P1≡_(merge-eq) P2 iff

-   -   name(P1)≡name(P2)    -   width(P1)=width(P2)    -   B_(u)(P1)=B_(u)(P2)

where P1 and P2 are user-binned coverpoints and B_(u)(P) indicates theset of user defined bins.

1.1.2.2 Flexible Merge Equivalence

P1≡_(merge-eq) P2 iff

-   -   name(P1)≡name(P2)    -   width(P1)=width(P2)

Note that the requirement to have same set of user-defined bins isdropped.

1.2 Merge Equivalence of Crosspoints

Consider a crosspoint,

C=P1×P2 . . . . ×Pn

We define the following:

B_(u)=Set of user defined bins in cross C

For bin bεB_(u), we define the following:

S_(b)=Auto cross bins comprising the user defined bin b

B_(a)=Set of auto cross bins=πΠB_(j)−S_(Bu)

B_(ign)=Set of illegal/ignore/unreachable auto cross bins

Π=Set cross product operator

B_(j)=Set of bins in jth coverpoint, P_(j).

The notation B_(u) (C) is used to indicate the set of user defined binsfor cross C.

Similar notation is extended for other sets like B_(a) etc.

Consider two cross points:

C1=A1×A2 . . . ×An, where A1, A2, . . . , An are component coverpoints.

C2=B1×B2 . . . ×Bm, where B1, B2, . . . , Am are component coverpoints

1.2.1 Default Merge Equivalence

C1≡_(merge-eq) C2 if the following holds.

-   -   Both crosses have the same number of coverpoints.    -   n==m    -   Component coverpoints are pair wise merge equivalent.    -   A1≡_(merge-eq) B1, A2≡_(merge-eq) B2 . . . , An≡_(merge-eq)Bm    -   B_(u)(C1)=B_(u)(C2)    -   If D is the set of user defined bins in C1 and C2, then the        following holds ∀dεD S_(d)(C(T1))=S_(d)(C(T2))

1.2.2 Flexible Merge Equivalence

C1≡_(merge-eq) C2 if the following holds.

-   -   Both crosses have the same number of coverpoints.    -   n==m    -   Component coverpoints are pair wise merge equivalent.    -   A1≡_(merge-eq) B1, A2≡_(merge-eq) B2 . . . , An≡_(merge-eq) Bm

The above rules have the following implication:

-   -   If a coverpoint cannot be merged, any crosspoint involving that        coverpoint will simply be added to the merged log. In        short≠_(merge-eq) of coverpoints has a ripple effect to all the        crosspoints.

1.2 Merge Equivalence of Assertions

1.2.1 Default Merge Equivalence

For assertions A1 and A2, A1≡_(merge-eq) A2 if the following holds.

name(A1)=name(A2)

1.2.2 Flexible Merge Equivalence

These rules are same as default merge equivalence.

2. Rules for Merging

The rules for default merging are straightforward since default mergeequivalence ensures that the coverage space is the same across logsbeing merged.

The following sections describe the rules for flexible merging.

2.1 Rules for Merging Coverpoints and Crosspoints

Let M(T1) be a coverpoint/crosspoint in T1 and M(T2) be acoverpoint/crosspoint in T2.

1. If M(T1)≡_(merge-eq) M(T2), after merging according to the rulesdefined above, bins defined only in the older functional coverage modelwill be dropped.

2. If M(T1)≠_(merge-eq) M(T2) then the merged log contains M(T2), whereT2 is the log with a more recent functional coverage model.

2.2 Rules for Merging Assertions

If A(T1) is an assertion in log T1 and A(T2) is an assertion in log T2,then they are merged according to the following rules.

1. If A(T1)≡_(merge-eq) A(T2) then the merged log contains A(T1)(orA(T2)).

2. If A(T1)≠_(merge-eq) A(T2) then the merged log contains A(T2), whereT2 is the more recent log.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims.

1. An electronic design automation method, comprising: using a computer,merging a first coverage log of a first simulation of a hardwaredescription language circuit design with a second coverage log of asecond simulation of the hardware description language circuit design,the first simulation being based on a first hardware verificationlanguage coverage model of the hardware description language circuitdesign, the second simulation being based on a second hardwareverification language coverage model of the hardware descriptionlanguage circuit design, the second hardware verification languagecoverage model being newer and different than the first hardwareverification language coverage model.
 2. The electronic designautomation method of claim 1, wherein a hardware description language ofthe hardware description language circuit design includes any ofVerilog, SystemVerilog, and VHDL.
 3. The electronic design automationmethod of claim 1, wherein a hardware verification language of the firstand second hardware verification language models includes any ofSystemVerilog, Native Testbench, E, and Vera.
 4. The electronic designautomation method of claim 1, wherein said merging includes: responsiveto the first coverage log and the second coverage log includingdifferent maximum numbers of automatically created bins, retainingcoverage data of the cover point of the second coverage log and deletingcoverage data of the cover point from the first coverage log.
 5. Theelectronic design automation method of claim 1, wherein said mergingincludes: responsive to the first coverage log including a bin having abin name absent from the second coverage log, deleting coverage data ofthe bin after said merging.
 6. The electronic design automation methodof claim 1, wherein said merging includes: responsive to the firstcoverage log including a bin having a bin name present in the secondcoverage log, retaining coverage data of the bin after said merging. 7.The electronic design automation method of claim 1, wherein said mergingincludes: responsive to the first coverage log having a cover point witha first expression width and the second coverage log having the coverpoint with a second expression width different from the first expressionwidth, retaining coverage data of the cover point with the secondexpression width and deleting coverage data of the cover point withfirst expression width.
 8. The electronic design automation method ofclaim 1, wherein said merging includes: responsive to the first coveragelog having a bin with a first bin definition and the second coverage loghas the bin with a second bin definition different from the first bindefinition, then after said merging, retaining coverage data of the binwith the second bin definition and deleting coverage data of the firstbin definition.
 9. The electronic design automation method of claim 1,wherein said merging includes: responsive to the first coverage logincluding a cross coverage point name of a cross coverage point namingat least two coverage point identifiers, and the second coverage logincludes the cross coverage point name of the cross coverage pointnaming said at least two coverage point identifiers, then deletingcoverage data of the cross coverage point of the first coverage logresponsive to deletion of coverage data of at least one of theidentified coverage points from the first coverage log.
 10. Theelectronic design automation method of claim 1, wherein said mergingincludes: responsive to the first coverage log including a firstuser-defined cross bin of a first cross coverage point, and the secondcoverage log including a second user-defined cross bin of a second crosscoverage point, the first cross coverage point and the second crosscoverage point having a same name, then despite the first user-definedcross bin and the second user-defined cross bin having differentdefinitions, retaining coverage data of the first user-defined cross binand coverage data of the second user-defined cross bin.
 11. Theelectronic design automation method of claim 1, wherein said mergingincludes: responsive to the first coverage log including first autocrossbins of a first cross coverage point, and the second coverage logincluding a second user-defined cross bin subsuming the autocross binsof the first cross coverage point, retaining coverage data of the firstautocross bins in the second user-defined cross bin of the secondcoverage log.
 12. The electronic design automation method of claim 1,wherein said merging includes: responsive to the first coverage logincluding a first cross coverage point, and the second coverage logincluding a second cross coverage point, then despite the first crosscoverage point and the second cross coverage point having different autocross coverage space, retaining i) auto cross coverage data of the firstcross coverage point that is subsumed by auto cross coverage space ofthe second cross coverage point, and ii) coverage data of the secondcross coverage point.
 13. The electronic design automation method ofclaim 1, wherein said merging includes: responsive to the secondcoverage log including a cross coverage point absent from the firstcoverage log, retaining the cross coverage point.
 14. The electronicdesign automation method of claim 1, wherein said merging includes:responsive to the first coverage log including a cross coverage pointabsent from the second coverage log, deleting the cross coverage point.15. The electronic design automation method of claim 1, wherein saidmerging includes: responsive to the first coverage log includingproperty coverage absent from the second coverage log, retaining theproperty coverage.
 16. The electronic design automation method of claim1, wherein said merging includes: responsive to the second coverage logincluding property coverage absent from the first coverage log,retaining the property coverage.
 17. A non-transitory computer readablemedium with computer readable instructions, which when executed by acomputer perform an electronic design automation method for circuitdesign, comprising: computer instructions merging a first coverage logof a first simulation of a hardware description language circuit designwith a second coverage log of a second simulation of the hardwaredescription language circuit design, the first simulation being based ona first hardware verification language coverage model of the hardwaredescription language circuit design, the second simulation being basedon a second hardware verification language coverage model of thehardware description language circuit design, the second hardwareverification language coverage model being newer and different than thefirst hardware verification language coverage model.
 18. An apparatus,comprising: a computer with a memory storing computer instructionsmerging a first coverage log of a first simulation of a hardwaredescription language circuit design with a second coverage log of asecond simulation of the hardware description language circuit design,the first simulation being based on a first hardware verificationlanguage coverage model of the hardware description language circuitdesign, the second simulation being based on a second hardwareverification language coverage model of the hardware descriptionlanguage circuit design, the second hardware verification languagecoverage model being newer and different than the first hardwareverification language coverage model.